Aug 11, 2015
Space Odyssey

Sen—In Greek mythology, Tethys was a Titan, one of the children of Gaia and Uranus. She was a sea goddess. Her brother, Cronus, overthrew Uranus to become King of the gods, until his own son Zeus overthrew him.

When the Romans decided to culturally appropriate the Greek gods, Cronus became Saturn. When astronomers started using the names of gods as planets and their associates as moons, the moniker Tethys was hung on one of Saturn’s icy satellites, a respectable world over 1000 km in diameter.

Odysseus was the protagonist of Homer’s Odyssey, an epic poem about the man’s travels back to his home in Greece after the Trojan War. Most of that journey was spent on the seas.

So it is appropriate, I think, that the largest impact feature on the Saturnian moon Tethys is named Odysseus. And by large, I mean overwhelmingly huge. The image above shows Odysseus on the edge of the crescent Tethys, taken by the Cassini spacecraft on May 9, 2015.

Odysseus is about 450 km across, a large fraction of the size of the moon itself, and twice the size of the impact crater in Mexico that resulted in the demise of the dinosaurs.

Ye gods.

Whatever hit there eons ago was huge. Odysseus is so big its floor is curved, following the spherical shape of the moon, relaxing into that shape as the water flowed after the impact. Had the impactor been much bigger, or moving faster, Tethys might have shattered under the massive blow. In fact, given its size, it probably would have shattered a solid icy body, implying that Tethys had an undersurface ocean or was at least partially molten at the time.

An aside: Although there’s no rigid and rigorous definition of the term, impact craters that size are called basins.

The Cassini image was made using three separate pictures composited into one. Sometimes blue, green, and red sub-images are used to mimic what our eyes see, but in this case what you see as blue is actually ultraviolet, green is actually green, and red is infrared. This is not how you would see Tethys if you were floating above it, but it does give better insight into surface features that might otherwise be invisible using visible colors.

Most obviously, the crater floor is brighter than the surrounding terrain. I can think of three reasons that might be: One is that the surface texture in the impact basin is different than the material around it, reflecting light better. Another is that the surface composition is different; the impact may have dredged up material from under the surface that’s brighter. Related to that is that exposure to ultraviolet light and subatomic particle impacts has darkened the surface of the moon, and the crater is younger enough that it hasn’t been modified.

Whatever the cause it makes for a very dramatic photo. You can also see, very faintly, the “dark side” of Tethys in this photo; the bright crescent is illuminated by the Sun, but the night side is being illuminated by Saturn.